The invention relates to self-powered radiation detectors such as are used for nuclear reactor flux measurements. More particularly the invention relates to gamma sensitive self-powered detectors which are designed for in-core reactor flux measurements. The conventional self-powered detector includes a centralized conductive emitter, a dense metal oxide insulator disposed about the central emitter and a conductive collector sheet electrode disposed about the dense insulator. Such self-powered radiation detectors are described in U.S. Pat. Nos. 3,787,697 and 3,872,311.
A self-powered detector is one in which no operating potential be applied across the sensing electrodes. The output signal arises from the difference in the neutron or gamma response between the emitter and the collector electrodes. Recent developments have centered on gamma response self-powered detectors because of the fast response time of the device, and because of the slow burnup of the emitter material when disposed within the reactor core due to secondary effects of neutron absorption. These properties make gamma responsive self-powered detectors particularly suitable for satisfying Nuclear Regulatory Commission regulations requiring permanent in-core safety monitoring of large size reactors. Most such gamma responsive self-powered detectors utilize platinum as the emitter material with a dense aluminum oxide particulate insulator disposed about the platinum, and an Inconel steel sheath or collector electrode about the insulator. In a gamma field, electrons are forced out of the emitter towards the collector and also out of the collector towards the emitter. The net response of the detector consists of the difference between these currents. For a typical prior art self-powered detector of conventional length, a current of approximately 5 .times. 10.sup.-8 amperes would be produced in a gamma flux field of 10.sup.8 Roentgens per hour. Such a current level is very difficult to measure in the reactor environment, because of the electrical noise which is typically present. In addition, error signals arise since the coaxial signal cables which connect the in-core detector to the exterior of the reactor and into the control panel area give rise to generation of error current signals from the gamma field. This is because the signal cable has the essential configuration of the self-powered detector itself and an error signal arises because of the differing response of the central conductor and the sheath conductor. It is possible to reduce or compensate for such signal cable gamma error signals, but in general it is desirable that the self-powered detector have an improved gamma sensitivity and a generally higher signal level.
Since such self-powered radiation detectors are designed for in-core reactor use there are practical overall outside diameter constraints imposed upon the detector design. These impose a practical limit upon the dimensioning of the emitter diameter which might be expected to offer some increase in sensitivity of the device. A fairly conventional overall outer diameter constraint of about 0.150" has become standard. It turns out that the current from gamma sensitive self-powered detectors increases very slowly with increasing emitter diameter. This observation along with the tight diameter size constraints imposed upon in-core detectors means that increasing the emitter diameter is not a viable method of improving the device response.
A method of significantly increasing the current from such self-powered detectors emerges from a detailed understanding of the current producing mechanism for such devices. The current from self-powered detectors is made up of the difference between two components. A positive current component is had from electrons reaching the collector from the emitter, and a negative component from electrons reaching the emitter from the collector. It has been discovered that the presence of the dense insulating material between the emitter and the collector tends to decrease the positive component and increase the negative component, thus resulting in a lower net detector response than would occur without the insulator. The electrons from the electrodes which stop in the insulator give rise to an electric field which causes return of some electrons to their electrode of origin and advances some electrons to the other electrode. For electrons from the emitter, some will stop in the insulator and return to the emitter and will not contribute to device current. Some electrons from the collector not heading toward the emitter will be stopped in the insulator, but will be advanced to the emitter by the electric field from the insulator. In the absence of an insulator those electrons not heading directly toward the emitter would miss it and hence not contribute to the current. The dense solid insulator tends to decrease the positive component and increase the negative component, resulting in lower net current.